Sharp reduction in nutrient concentrations in deep British Columbian strait linked to marine heatwave Hayley V. Dosser1, Stephanie Waterman1, Jennifer Jackson2, Brian Hunt1,2 1- University of , Vancouver, 2- Hakai Institute, Victoria & Heriot Bay

1. Key Points 3. Nutrient Concentration Decline 4. Sampling Bias and Inter-agency Comparison

A marine heatwave known as The Blob and a subsequent El Nino˜ The 2015-2018 mean concentrations of all three nutrients declined by To confirm the decline in nutrient concentrations during 2015-2018 is a impinged on the British Columbian coast from 2014 to 2016. From 2015 roughly two standard deviations from their 1975-2014 mean values. real signal, we must first rule out other causes: onwards, anomalously low levels of nutrients were recorded in I Seasonal bias due to differences in the months sampled. Johnstone Strait, a 400m deep channel north of . Low I Discrepancies between Hakai and DFO data collection and levels have persisted through 2018. processing.

2. Study Area and Data Since 2015, most nutrient samples in Johnstone Strait have been collected by the Hakai Institute during the spring and summer.

Figure 2: Probability Seasonal bias: Figure 1: distributions for We compare data from before 2015 with data from 2015-2018 using only Bathymetric map for nitrate+nitrite, samples from the months common to both periods: April, May, Johnstone Strait phosphate, and September, and November. and nearby regions. silicate nutrient This study focuses concentrations from on data from the before 2015 (in Inter-agency comparison: deeper western end blue) and from Bottle samples were independently of Johnstone Strait, 2015-2018 (in red). collected by scientists from both from the sill with Dark purple DFO and Hakai. Queen Charlotte indicates overlap Strait to Kelsey Bay. between the We have one point of comparison distributions. in July 2017, which shows good agreement for all 3 nutrients, validating the result.

Figure 3: Vertical profiles of nitrate concentration collected by DFO (in blue) on July 6th, 2017 and by Hakai (in red) on July 4th, 2017 at nearby sampling stations.

5. Nutrients and Upwelling 6. The Blob and El Nino˜ in Johnstone Strait 7. Upwelled water on the continental shelf

Nutrients in Johnstone Strait are replenished each spring by upwelling of Comparing properties in Johnstone Strait during the spring upwelling Queen Charlotte Sound is the main upwelling path to Johnstone Strait. dense, cold, saline water arriving through Queen Charlotte Sound. period shows anomalously warm, fresh, and low-density water during The Blob and El Nino˜ years. Upwelled water on the shelf does not show a decline in nutrients during 2015-2018. Nutrient Figure 5: Probability concentration distributions using increases with data from April and density, Figure 4: Lateral density transect along the thalweg (bathymetry in grey) from Queen May for potential suggesting a Charlotte Strait (QCSt), through western Johnstone Strait (JS), eastern JS and the temperature, reduction in (MDI), and into the (SoG). White arrows show likely salinity, and dense, upwelled circulation and pink arrows indicate tidal mixing. potential density water passing anomalies from over the sill from before 2015 (in QCSt into JS. Hypothesis: blue) and from 2015-2018 (in red). Anomalously warm, low-density water from The Blob and El Nino:˜ Figure 6: Nitrate vs. density along a sampling line in Queen Charlotte Sound, with Dark purple pre-2015 samples in blue and 2015-2018 in red. Inset shows bathymetric map. I depress deep isopycnals on the continental shelf indicates overlap I limit or delay the inflow of nutrient-rich upwelled water passing over between the 8. Summary the sill from into Johnstone Strait distributions. I reduce volume of nutrient-rich water entering Johnstone Strait During The Blob and El Nino˜ years, and continuing into 2018, Johnstone Strait experienced an inflow of anomalously warm, fresh, low-density water during spring upwelling. Nutrient concentrations were El Nino˜ is known to cause weak coastal upwelling along the British anomalously low from 2015-2018; we hypothesize this is due to less Columbian coast, with anomalously shallow, warm, and fresh upwelled water making it over the sill into the Strait as dense, upwelled source water. nutrient-rich waters are pushed deeper in the water column. [email protected] Funding for this research provided by MEOPAR, Mitacs, and the Tula Foundation